When Instagram first came out, I thought it was pretty lame and was just a way for mobile photographers to add filters to make their lousy photos look better. A while back, I signed up and was surprised to see National Geo and others posting real (i.e. not mobile) photos. After realizing that you can upload your own photos, I began doing so and just uploaded my 99th photo today. I don't like the square format - I don't like 4:3, either, I'm definitely a 3:2 or wider guy, but I'm having fun with it. I like seeing others photos and sharing my work with people from around the world.
First, I applaud you for having an open mind and checking it out despite your initial assumptions/reservations. Judging from many posts on this forum and other photography-themed ones, it seems to me many photogs tend to be a bit close-minded and not likely to change opinions on many subjects.
I haven't used Instagram in over year, mainly due to it being a big time-suck if I let it (not only browsing other's photos, but I tried to respond to each and every person's comments on my pics, which led me to start dreading posting...haha). That, and having several pictures 'stolen' with other users posting them as their own pictures. Not that I sell my work, but just having them stolen was frustrating and annoying.
Other than that my experience was extremely positive, probably partly because I joined before it blew up into a selfie/celebrity popularity contest. I actually had pics hit the popular page a number of times before it became full of selfies and had a few thousand followers, most of whom were actually interested in photography. That's not any big statement on my pictures or ability as I'm still just an amateur, I'm just saying things were different a couple of years ago.
Instagram is different things to different people, but there are serious photographers on there, pros and amateurs. I used to get feedback on my work which drove me to try to get better (at least a little ) and to connect with other photogs, just as it sounds like you're doing. My interest is mainly landscape, and after I got past the fear of meeting potentially psychotic strangers, I joined a few photo meetups and even led a couple (California coast and Antelope Canyon). Along the way I've been extremely fortunate to have made some great friends, one of whom plans 1-2 trips a year with me with no end in sight as we're trying to check things off our bucket lists.